Multi-site TBT binding skews the inhibition of oligomycin on the mitochondrial Mg–ATPase in Mytilus galloprovincialis

Tributyltin (TBT), a persistent lipophilic contaminant found especially in the aquatic environment, is known to be toxic to mitochondria with the F1F0–ATPase as main target. Recently our research group pointed out that in mussel digestive gland mitochondria TBT, apart from decreasing the catalytic efficiency of Mg–ATPase activity, at concentrations ≥1.0 μM in the ATPase reaction medium lessens the enzyme inhibition promoted by the specific inhibitor oligomycin. The present work aims at casting light on the mechanisms involved in the TBT-driven enzyme desensitization to inhibitors, a poorly explored field. The mitochondrial Mg–ATPase desensitization is shown to be confined to inhibitors of transmembrane domain F0, namely oligomycin and N,N′-dicyclohexylcarbodiimide (DCCD). Accordingly, quercetin, which binds to catalytic portion F1, maintains its inhibitory efficiency in the presence of TBT. Among the possible mechanisms involved in the Mg–ATPase desensitization to oligomycin by ≥1.0 μM TBT concentrations, a structural detachment of the two F1 and F0 domains does not occur according to experimental data. On the other hand TBT covalently binds to thiol groups on the enzyme structure, which are apparently only available at TBT concentrations approaching 20 μM. TBT is able to interact with multiple sites on the enzyme structure by bonds of different nature. While electrostatic interactions with F0 proton channel are likely to be responsible for the ATPase activity inhibition, possible changes in the redox state of thiol groups on the protein structure due to TBT binding may promote structural changes in the enzyme structure leading to the observed F1F0–ATPase oligomycin sensitivity loss.

► Tributyltin (TBT) is lipophilic, toxic to mitochondria and especially to F0F1complex.
► High TBT concentrations (≥1 μM) in vitro desensitize F0F1 to F0 inhibitors in mussel.
► TBT covalently binds to thiol groups in a putative low-affinity site on the enzyme.
► TBT binding may trigger changes yielding F1F0 refractoriness to F0 inhibitors.